1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring a rate of distortion of a measured material having light transmission properties such as a flat glass plate, a curved glass plate or the like. In particular, the present invention relates to an improvement of a method of and an apparatus for measuring optical distortion by measuring a rate of distortion on the basis of light transmitting through a measured material having light transmission properties.
2. Discussion of Background
Generally, when a window glass for an automobile or a building has a large rate of distortion, one sees a distorted outer sight through the window glass. Accordingly, it is necessary that a rate of distortion of window glass sheets should be in a predetermined range by quality control.
Heretofore, as techniques for measuring the distortion of window glass sheets, there are as follows for instance, (1) a regular pattern composed of polka dots is used, and the optical distortion of a measured material is calculated by evaluating the diameters in the longitudinal direction and lateral direction of polka dot data obtained by taking a picture of the measured material such as a window glass sheet, or, (2) a regular pattern composed of a group of points marked by laser beams, for instance, which are arranged at predetermined distances of pitch along the longitudinal direction and the lateral direction, is used, and the optical distortion of a measured material is calculated by detecting a change of distance in coordinates of scattered bright spots of the group of points in a picture taken through the measured material.
As another technique of measuring the distortion of the window glass sheet of this kind, there is a technique disclosed in, for instance, Japanese Unexamined Patent Publication No. 199946/1991. The publication proposes that a screen on which a predetermined bright line is irradiated is disposed in rear of a measured material such as a window glass sheet or the like, and an imaging device such as an ITV camera is disposed in front of the measured material whereby a picture of the bright line is taken through the measured material in the entire region of the measured material with the imaging device, and the optical distortion of the measured material is measured from data in the picture.
However, in the method of evaluating the diameters in the longitudinal direction and the lateral direction of the polka dots, for instance, it is necessary to precisely detect the central position of each of the polka dots in order to correctly obtain the diameters in the longitudinal and lateral directions. Accordingly, the treatment of measuring the optical distortion becomes complicated and it takes much time for the treatment because there are restrictions such as the resolution of the imaging device when the ITV camera is used.
On the other hand, in the method of using the regular pattern composed of a group of points, the optical distortion is obtained on the basis of the distances between points. Accordingly, the data of distances of the points should have a fairly high density in order to measure the optical distortion correctly. However, it is difficult to increase a range to be measured of the measured material when the imaging device is used. On the contrary, when the density of data of the distances of points is made thin, there is a risk of overlooking of the distortion of a local portion between points.
In either method of measuring the optical distortion, a picture of the polka dots or the group of points is taken through the measured material while the imaging device such as the ITV camera or the like is moved. Therefore, error of measured distance is apt to occur due to mechanical vibrations caused when the imaging device is moved. Further, since a range of picture taken by the imaging device by once time is small, it is necessary to repeat a large number of times of movement and treatments for picture. Accordingly, it takes much time for the process.
When the imaging device is used, there occasionally causes blooming or out of focusing. Therefore, error of dimensions of the diameters in the longitudinal and lateral directions of the polka dots becomes large. Further, error of the position of dispersed bright point coordinates of the group of points becomes large. As a result, accuracy of measuring of the optical distortion will decrease.